Coordination points provide a lock mechanism to determine which nodes get to fence off data drives from other nodes. A node must eject a peer from the coordination points before it can fence the peer from the data drives. SFCFSHA prevents split-brain when vxfen races for control of the coordination points and the winner partition fences the ejected nodes from accessing the data disks.
The coordination points can either be disks or servers or both.
Coordinator disks
Disks that act as coordination points are called coordinator disks. Coordinator disks are three standard disks or LUNs set aside for I/O fencing during cluster reconfiguration. Coordinator disks do not serve any other storage purpose in the SFCFSHA configuration.
You can configure coordinator disks to use Veritas Volume Manager's Dynamic Multi-pathing (DMP) feature. Dynamic Multi-pathing (DMP) allows coordinator disks to take advantage of the path failover and the dynamic adding and removal capabilities of DMP. So, you can configure I/O fencing to use DMP devices. I/O fencing uses SCSI-3 disk policy that is dmp-based on the disk device that you use.
See the Storage Foundation Administrator's Guide.
Coordination point servers
The coordination point server (CP server) is a software solution which runs on a remote system or cluster. CP server provides arbitration functionality by allowing the SFHA cluster nodes to perform the following tasks:
Self-register to become a member of an active SFCFSHA cluster (registered with CP server) with access to the data drives
Check which other nodes are registered as members of this active SFCFSHA cluster
Self-unregister from this active SFCFSHA cluster
Forcefully unregister other nodes (preempt) as members of this active SFCFSHA cluster
In short, the CP server functions as another arbitration mechanism that integrates within the existing I/O fencing module.
Multiple SFCFSHA clusters running different operating systems can simultaneously access the CP server. TCP/IP based communication is used between the CP server and the SFCFSHA clusters.